Daylight harvesting is an available lighting strategy designed to reduce excessive internal light levels during peak consumption hours, wherein external light sources, such as daylight, substitute for interior electrical lighting. For example, in an office setting, each work area must at all times be provided with a desired level of light determined based upon the tasks performed in the area or zone. Lighting, however, is generally installed by size and number sufficient to provide the minimum light level under the assumption that no other light sources are available in the interior space. Yet, during varying times of the day, other light sources may illuminate the interior space such that the level of light present is excessive. Therefore, the use of interior lighting at the same level of intensity becomes a waste of energy.
Specifically, during the day, sunlight may enter through windows and skylights. When these external light sources are present, the preset brightness of interior lighting is not necessary since these external light sources provide some or all of the minimum light level required. Daylight harvesting eliminates the excessive level of intensity of interior lighting, conserving as much as 84% of the energy required to light a facility at the minimum light level. As such, bright sunlight is enabled to provide up to 100% of illumination during midday, when energy costs are highest.
Daylight harvesting also enables a constant level of light on work surfaces to avoid moments when the external light sources provide an excessive amount of light, resulting in periods of glare. In the alternative, when light levels are low (i.e. when clouds roll in or nighttime falls), daylight harvesting maintains this constant level of light by continuously increasing and decreasing the power provided to the internal lighting. This practice enables the worker to resolve images with ease. As a result, eyestrain is avoided; and health and productivity are promoted.
Conventional technology for implementing daylight harvesting techniques incorporates the use of digital photo-sensors to detect light levels, wherein the digital photo-sensor is connected to a dimmer control circuit to automatically adjust the output level of electric lighting for promotion of a lighting balance. Dimmer control circuits, as implemented with respect to daylight harvesting, gradually increase or decrease interior lighting in response to photocell measurement of ambient light levels.
In general, dimmer control systems are widely used in indoor lighting to provide a softer feel and more controllable illumination experience as compared to on/off lighting. Conventional dimmer control circuits include on/off switching and up/down power controls. Further, a microprocessor may be incorporated within a dimmer control circuit to provide control for various power-up, power-down and fade in/out functions. Rather than use a variable resistor type rheostat which wastes power and generates heat at low illumination levels, modern dimming control circuits employ phase regulation, in which the power circuit is switched on at a time delay following a zero-crossing of the AC sine wave input until the end of each half cycle, in an effort to supply a variable level of power to the lighting load.
When commissioning a lighting system installation that employs daylight harvesting techniques, the target voltage level for the minimum level of light or the desired steady state light level must be established for the light sensor which conventionally is a standard 0-10 volt photocell. This target voltage level is generally set manually. Accordingly, this approach requires a user to monitor the system after sunset and record the lowest value read. Thereafter, the user is required to set the desired steady state light level according to this value.
This approach, however, is not cost efficient nor convenient since it employs the use of staff or personnel after hours.
Thus, a need exists for a better calibration technique than has been described above for determining the target voltage level of a light sensor included within a light control system having a daylight harvesting feature.